fate: workaround for slighly broken 'test' shell builtin

Some shells, e.g. minix3, have a broken 'test' builtin which fails
if the first operand of a binary operator looks like a unary operator.
Prefixing the values with 'x' prevents this from happening.
Signed-off-by: Mans Rullgard <mans@mansr.com>

tests/fate-run.sh

@@ -78,7 +78,7 @@ avconv(){
     dec_opts="-threads $threads -thread_type $thread_type"
     avconv_args="-nostats -cpuflags $cpuflags"
     for arg in $@; do
-        [ ${arg} = -i ] && avconv_args="${avconv_args} ${dec_opts}"
+        [ x${arg} = x-i ] && avconv_args="${avconv_args} ${dec_opts}"
         avconv_args="${avconv_args} ${arg}"
     done
     run avconv ${avconv_args}